1. Field of the Invention
The present invention relates to an electro-photographic type developing apparatus and method. More particularly, the invention relates to a hybrid type developing apparatus and method which uses a magnetic carrier and a nonmagnetic toner.
2. Description of the Related Art
Developing methods for image forming apparatuses are known using electro-photography such as copying machines, printers, facsimiles and multi-function apparatuses. One method is a dual component developing method using a toner and a carrier. A mono component developing method uses an insulation toner or a conductive toner. A hybrid developing method uses nonmagnetic toner that is charged by rubbing with magnetic carrier, and only charged toner is attached onto a development roller. In each method, the toner is supplied to an electrostatic latent image, thereby developing the electrostatic latent image.
The dual component developing method has advantages of excellent charging of the toner, durability, realization of uniform beta images, and so on. However, the dual component developing method requires a bigger and more complicated apparatus, scattering of the toner to allow a carrier to attach to a latent image, which results in image-quality deterioration due to a lack of durability of the carrier, and so forth.
The mono component developing method is advantageous since a compact developing apparatus can be used and excellent dot reproduction can be obtained. This method has the disadvantages of lower durability due to the deterioration of a development roller and a charging roller, higher costs due to the need to exchange the developing apparatus itself when toner is exhausted, occurrence of selective development, and so on. Selective development occurs when only the toner having a desired weight and charge is moved to the latent images from the developing roller. If the selective development occurs, because toner having a lower weight than the desired weight and a smaller charge than the desired charge can not be used for development, usage ratio may decrease.
The hybrid developing method has advantages of excellent dot reproduction, durability, and high speed image formation. However, development ghosts occur if the toner supplied to the development roller is insufficient or toner on the development roller is not fully removed after development. The occurrence of development ghosts will now be briefly explained with reference to FIG. 1. Referring to FIG. 1, a toner layer formed on the surface of the development roller has an area Ai facing an image portion of an image receptor which is developed on the image receptor by a development bias. An area Ab facing a non-image portion remains on the development roller without being developed. At this time, the amount of toner developed from the area Ai on the image receptor is denoted as Ma. New toner is supplied to the development roller for subsequent development. If the amount of toner supplied to the development roller is less than Ma, the thickness of the toner layer formed on the surface of the development roller becomes non-uniform, and a development ghost in which a latent image of an earlier development process remains in a subsequent development process occurs. Such a development ghost more easily occurs in continuous printing.
To solve these problems, a DC bias (Japanese Unexamined Patent Application Publication No. 7-72733) superposed on a DC bias or an AC bias (Japanese Unexamined Patent Application Publication Nos. 6-67546 and 7-92804) is applied to a magnetic roller. The polarity of the DC bias applied to the magnetic roller is switched when image formation is complete at regular intervals to provide an electric field in a proper direction for recovering toner from the development roller to the magnetic roller. Since a lot of time is required to form a toner layer having proper thickness on the development roller in subsequent development, such a method is unsuitable for high speed printing.
When an electrode is installed between the development roller and a photo conductor, a non-uniform development arises due to vibration of a wire tighten by an electrical bias or stripe traces are formed on the development roller due to dust instantly attached to the electrode. An example of a development roller in which an electrode is covered so as to prevent such phenomena is disclosed in Japanese Unexamined Patent Application Publication No. 2000-250294.